Sorry for long reply was a little busy. Opening same book I got something that looked similar to yours, but I saw screen having darker shades in menus etc. As a result to compare contrast I took pictures using my SLR camera with exactly the same white balance, exposure and same light conditions on screens that showed the darkest blacks. Neither my kobo or ereader had a backlight on.
I opened the images in photoshop and used the color dropper to get the colours of the blacks and the whites. I can see a very minuscule difference with eyes but the dropper shows it clearer. The whites have a small tint towards the yellows and the blacks are indeed very slightly darker.
Energy E-Reader Pro+
Kobo Touch
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Heres two pictures of the ebook you were using as well. Black shade is almost identical and the white is same as above - a more yellowy tint to it.
Energy E-Reader Pro+
Kobo Touch
I'm quite happy with mine and can see there is a tiny difference, pity I don't have a Boyue to test for you. Maybe they have changed the OS slightly and it behaves different.
Try a pure black picture in a gallery app on both perhaps and also a white picture. Best of luck with the support!